Fire doors for commercial buildings must meet certain local building standards, depending upon nation, state and other level of jurisdiction regulations. In general, fire doors must withstand a given level of heat on one face for a given amount of time. During this time, the door must also block passage of smoke around its periphery, maintain structural strength and so on.
Thus, doors may be rated as “60 minute doors”, “20 minute doors”, or other types of ratings depending upon the testing organization and local regulations.
However, obtaining this rating is an expensive and time consuming process. A door manufacturer must first construct the door. The interior of the door is normally a laminate or series of layers of various types of structural and fire resistant materials, this interior is called the “core”. After the door manufacturer has made the core, they then add panels, runners and other material to the outside facings of the door to provide it with an appearance suitable for commercial use: expensive wood materials, 6 panel arrangements, spots for name plates and so on. Then the manufacturer sends the door to the rating agency for testing. The rating agency will take the door, place it in what amounts to a large oven, and begin exposing one side to flame and extremes of heat. In one testing arrangement, after the door has passed the time rating (for example 60 minutes) without allowing excess heat to reach the “cool” side, it is then required to pass a structural test involving shooting water from a fire hose at the cool side to see if the door collapses under this heavy reverse load.
This testing requirement imposes costs and labor upon door manufacturers. In effect, each model of door must be individually certified to be fire resistant before it can be sold. This imposes a high barrier to entry into the market, since there is no provision for a door manufacturer to buy a “pre-certified” and pre-made core and use it to make an automatically certified door.
Sodium silicate is an extremely useful material for door manufacturers when they are attempting to make doors with the necessary fire resistance. Sodium silicate may be any one or a mixture of several compounds having silica (Si2O) and Sodium oxide (Na2O). Forms of “soluble glass” or “water glass” include sodium disilicate, sodium metasilicate, sodium orthosilicate, sodium tetrasilicate and so on. These may be produced by combining sand and sodium carbonate. Considered a non-hazardous substance (MSDS standards) it does require gloves for handling, as it is extremely destructive of human mucus membranes and may theoretically even cause burns on skin contact.
Importantly, sodium silicates have high melting points and are water soluble. More importantly, when exposed to heat, hydrated sodium silicate absorbs the heat and uses it for a phase transformation. Like water boiling from liquid to gas and maintaining a temperature of 100 degrees C. during the process, sodium silicate changes form, unlike water, the sodium silicate does not boil away or change from a liquid to a gas.
Exposed to heat, several energy absorbing changes occur in hydrated sodium silicate. Firstly, it begins to give up the water molecules trapped within it. The boiling away water molecules carry away a good deal of heat, but crucially the second effect is that the sodium silicate expands (intumesces) from a relatively solid form into a glass foam containing numerous cavities and pockets. This transformation itself absorbs heat and the result is a notably thicker mass of an excellent insulator having a melting point above 800 degrees C. (circa 1500 degrees Fahrenheit). The thickness change is very dramatic: a ratio of six to one expansion is possible.
Thus, door manufacturers frequently use sodium silicate when constructing doors, in particular while the door manufacturer is constructing the core of the door. Care is taken to ensure that the sodium silicate is held in place by materials which will allow its expansion during combustion, this in fact may be regulatory requirement in some jurisdictions.
However, sodium silicate has certain disadvantages. Most notably, exposed to heat it tends to lose structural strength. This means that a layer of sodium silicate within a door may begin to “slump” off of the door as the door burns away. By falling away or slumping away in a mass, the sodium silicate layer surrenders a great deal of its benefit. Door manufacturers must therefore use care in selecting how the sodium silicate layer of the door is supported by the adjacent layers.
However, common sense dictates that the sodium silicate be used as compactly as possible, with as low a mass of additional matter, so that door manufacturer is able to produce a light and not overly thick door.
The form of sodium silicate panel well known for use in fire door construction is available from only one major source, the European company BASF, which has an ancillary operation producing sodium silicate in the form of PALUSOL brand fire board. (Trademark of BASF, not associated with present applicants.) PALUSOL brand panel is a mixture of solid sodium silicate with a low percentage of dissociated strands of fiberglass fabric mixed in, sold in sheets large enough to individually span the width and height of a typical door. While the fiberglass strands provide some limited degree. of strength, BASF specification sheets for PALUSOL brand panel state that there is a risk of breakage is a single panel is transported by itself, and that it must be stored laid flat, and even that the peripheral 5 mm border of the sheet must be trimmed prior to actual use by a door manufacturer in construction of a door. Sodium silicate is normally translucent, the PALUSOL building product panels are white. This building product is produced by mixing into a large flat container of sodium silicate the loose fibers of fiberglass or organic fabric, then drying the hydrated sodium silicate until it assumes a solid cellular form with a degree of water still therein.
One PALUSOL brand fire panel produced by BASF includes a wire mesh at 25 millimeter intervals (almost exactly one inch squares between wires). This is necessary for the door manufacturers as an extended area of PALUSOL will simply slump under heat or even buckle while the door manufacturer is attempting to work with it.
A PALUSOL brand panel must be finished by coating of both sides with epoxy. The PALUSOL water content will otherwise evaporate over time, a tendency so marked that PALUSOL brand panel must even be coated at the edges with epoxy. Door manufacturers making cores are directed by the maker of the PALUSOL brand panels to re-epoxy any edges which are cut down during the course of manufacture. And despite all efforts and expense, panels of PALUSOL brand panel still degrade over time.
It would be preferable to provide a door core not requiring individual certification of each model of door by door manufacturers.
It would further be preferable to provide a product having better thermal protection than panels of sodium silicate with loose fiberglass therein.
It would further be preferable to provide sodium silicate panels having greater structural strength.
It would further be preferable to provide panels having a sodium silicate content which does not require as much effort in preservation and which lasts longer in practice.